1. Applied Discrete Mathematics Week 4: Number Theory
Congruences
Theorem: Let m be a positive integer. If a  b (mod m) and c  d (mod m), then a + c  b + d (mod m) and ac  bd (mod m).
Proof:
We know that a  b (mod m) and c  d (mod m) implies that there are integers s and t with b = a + sm and d = c + tm.
Therefore,
b + d = (a + sm) + (c + tm) = (a + c) + m(s + t) and
bd = (a + sm)(c + tm) = ac + m(at + cs + stm).
Hence, a + c  b + d (mod m) and ac  bd (mod m).
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

2. The Euclidean Algorithm
The Euclidean Algorithm finds the greatest common divisor of two integers a and b.
For example, if we want to find gcd(287, 91), we divide 287 (the larger number) by 91 (the smaller one):
287 = 913 + 14
3 = 14
(-3) = 14
We know that for integers a, b and c, if a | b, then a | bc for all integers c.
Therefore, any divisor of 91 is also a divisor of 91(-3).
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

3. The Euclidean Algorithm
(-3) = 14
We also know that for integers a, b and c,
if a | b and a | c, then a | (b + c).
Therefore, any divisor of 287 and 91 must also be a divisor of (-3), which is 14.
Consequently, the greatest common divisor of 287 and 91 must be the same as the greatest common divisor of 14 and 91:
gcd(287, 91) = gcd(14, 91).
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

4. The Euclidean Algorithm
In the next step, we divide 91 by 14:
91 = 146 + 7
This means that gcd(14, 91) = gcd(14, 7).
So we divide 14 by 7:
14 = 72 + 0
We find that 7 | 14, and thus gcd(14, 7) = 7.
Therefore, gcd(287, 91) = 7.
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

5. The Euclidean Algorithm
In pseudocode, the algorithm can be implemented as follows:
procedure gcd(a, b: positive integers)
x := a
y := b
while y  0
begin
r := x mod y
x := y
y := r
end {x is gcd(a, b)}
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

6. Representations of Integers
Let b be a positive integer greater than 1. Then if n is a positive integer, it can be expressed uniquely in the form:
n = akbk + ak-1bk-1 + … + a1b + a0,
where k is a nonnegative integer,
a0, a1, …, ak are nonnegative integers less than b,
and ak  0.
Example for b=10:
859 = 8  100
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

7. Representations of Integers
Example for b=2 (binary expansion):
(10110)2 = 124 + 122 + 121 = (22)10
Example for b=16 (hexadecimal expansion):
(we use letters A to F to indicate numbers 10 to 15)
(3A0F)16 = 3  160 = (14863)10
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

8. Representations of Integers
How can we construct the base b expansion of an integer n?
First, divide n by b to obtain a quotient q0 and remainder a0, that is,
n = bq0 + a0, where 0  a0 < b.
The remainder a0 is the rightmost digit in the base b expansion of n.
Next, divide q0 by b to obtain:
q0 = bq1 + a1, where 0  a1 < b.
a1 is the second digit from the right in the base b expansion of n. Continue this process until you obtain a quotient equal to zero.
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

9. Representations of Integers
Example: What is the base 8 expansion of (12345)10 ?
First, divide by 8:
12345 = 8
1543 = 8
192 = 824 + 0
24 = 83 + 0
3 = 80 + 3
The result is: (12345)10 = (30071)8.
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

10. Representations of Integers
procedure base_b_expansion(n, b: positive integers)
q := n
k := 0
while q  0
begin
ak := q mod b
q := q/b
k := k + 1
end
{the base b expansion of n is (ak-1 … a1a0)b }
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

11. Applied Discrete Mathematics Week 4: Number Theory
Addition of Integers
How do we (humans) add two integers?
Example: 1 1 1
carry 1 2 5 1 5 1 1
carry
Binary expansions: (1011) (1010)2 ( )2 1 1 1
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

12. Applied Discrete Mathematics Week 4: Number Theory
Addition of Integers
Let a = (an-1an-2…a1a0)2, b = (bn-1bn-2…b1b0)2.
How can we algorithmically add these two binary numbers?
First, add their rightmost bits:
a0 + b0 = c02 + s0,
where s0 is the rightmost bit in the binary expansion of a + b, and c0 is the carry.
Then, add the next pair of bits and the carry:
a1 + b1 + c0 = c12 + s1,
where s1 is the next bit in the binary expansion of a + b, and c1 is the carry.
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

13. Applied Discrete Mathematics Week 4: Number Theory
Addition of Integers
Continue this process until you obtain cn-1.
The leading bit of the sum is sn = cn-1.
The result is:
a + b = (snsn-1…s1s0)2
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

14. Applied Discrete Mathematics Week 4: Number Theory
Addition of Integers
Example:
Add a = (1110)2 and b = (1011)2.
a0 + b0 = = 02 + 1, so that c0 = 0 and s0 = 1.
a1 + b1 + c0 = = 12 + 0, so c1 = 1 and s1 = 0.
a2 + b2 + c1 = = 12 + 0, so c2 = 1 and s2 = 0.
a3 + b3 + c2 = = 12 + 1, so c3 = 1 and s3 = 1.
s4 = c3 = 1.
Therefore, s = a + b = (11001)2.
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

15. Applied Discrete Mathematics Week 4: Number Theory
Addition of Integers
procedure add(a, b: positive integers)
c := 0
for j := 0 to n-1 {larger integer (a or b) has n digits}
begin
d := (aj + bj + c)/2
sj := aj + bj + c – 2d
c := d
end
sn := c
{the binary expansion of the sum is (snsn-1…s1s0)2}
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

16. Applied Discrete Mathematics Week 4: Number Theory
Matrices
A matrix is a rectangular array of numbers.
A matrix with m rows and n columns is called an mn matrix.
Example:
is a 32 matrix.
A matrix with the same number of rows and columns is called square.
Two matrices are equal if they have the same number of rows and columns and the corresponding entries in every position are equal.
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

17. Applied Discrete Mathematics Week 4: Number Theory
Matrices
A general description of an mn matrix A = [aij]:
j-th column of A
i-th row of A
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

18. Applied Discrete Mathematics Week 4: Number Theory
Matrix Addition
Let A = [aij] and B = [bij] be mn matrices.
The sum of A and B, denoted by A+B, is the mn matrix that has aij + bij as its (i, j)th element.
In other words, A+B = [aij + bij].
Example:
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

19. Matrix Multiplication
Let A be an mk matrix and B be a kn matrix.
The product of A and B, denoted by AB, is the mn matrix with (i, j)th entry equal to the sum of the products of the corresponding elements from the i-th row of A and the j-th column of B.
In other words, if AB = [cij], then
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

20. Matrix Multiplication
A more intuitive description of calculating C = AB:
- Take the first column of B
Turn it counterclockwise by 90 and superimpose it on the first row of A
Multiply corresponding entries in A and B and add the products: 32 + 00 + 13 = 9
Enter the result in the upper-left corner of C
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

21. Matrix Multiplication
Now superimpose the first column of B on the second, third, …, m-th row of A to obtain the entries in the first column of C (same order).
Then repeat this procedure with the second, third, …, n-th column of B, to obtain to obtain the remaining columns in C (same order).
After completing this algorithm, the new matrix C contains the product AB.
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory

22. Matrix Multiplication
Let us calculate the complete matrix C: 9 7 8 15 15 20 -2 -2
February 13, 2018
Applied Discrete Mathematics Week 4: Number Theory